With a bit of license. We can extend the basic analogy by considering the sand in the upper chamber to be the stock of low-entropy fossil fuel on Earth. Depicted in the right-hand figure. Fossil energy is used at a rate determined by the constricted middle of the hourglass, but unlike a normal hourglass, humans alter the width (i.e., they change the rate of consumption of fossil fuels). Once consumed. The sand falls to the bottom of the chamber, where it accumulates as waste and interferes with terrestrial life processes.